This invention relates to a method of manufacturing a glass substrate for a magnetic disk that is mounted in a magnetic disk apparatus such as a hard disk drive and a method of manufacturing a magnetic disk.
A magnetic disk device represented by a hard disk drive (HDD) can record a larger amount of information as compared with other information storages. Further, since the information transmission speed is high, it has rapidly been spreading. A recording medium installed in the hard disk drive is a magnetic disk, which is manufactured by depositing a magnetic layer, used as a magnetic recording layer, on a disk-shaped substrate.
In a hard disk drive, a magnetic head performs high-speed scanning over the surface of a magnetic disk rotating at high speed while keeping a narrow flying height. Thus, recording/reproduction of information is carried out.
As a substrate for a magnetic disk, use is made of a metal substrate of an aluminum alloy or the like, a resin substrate of plastics or the like, or a glass substrate. Since the glass substrate has a higher rigidity as compared with the substrates made of other materials, it is advantageously suitable for high-speed rotation of a disk and excellent in shock resistance. Further, since the glass substrate can be finished so as to have an extremely smooth mirror surface by carrying out precision polishing, it is possible to further narrow the flying height of a magnetic head. Therefore, use of the glass substrate can realize a high information recording density.
Glass is a brittle material as a raw material but is also a material that can be chemically strengthened. When glass is chemically strengthened, it is possible to generate a strong compressive stress on the surface. Therefore, by applying predetermined chemical strengthening to a glass substrate, a sufficient strength can be given thereto in terms of a use in a magnetic disk.
As a manufacturing method of such a magnetic disk glass substrate, techniques as described in, for example, U.S. Pat. Nos. 5,654,057, 5,725,625, and 5,916,656 are known.
On the other hand, in recent years, a magnetic head mounted with a magnetoresistive effect (MR Effect, Magneto-Resistive Effect) type reproducing element has been adopted for the purpose of increasing an information recording density recordable on a magnetic disk. It is known that when this MR effect type reproducing element is used, a recorded signal can be reproduced with a high S/N ratio, while, thermal asperity defect is liable to occur. When the thermal asperity defect occurs, the reproduction of the recorded signal is impeded. U.S. Pat. Nos. 6,119,483, 6,427,489, 6,430,965, 6,523,367, and 6,534,120 each disclose a manufacturing method of a magnetic disk glass substrate that can reduce occurrence of the thermal asperity defect.
Recently, even higher information recording capacities have been required. For example, there is demand for a magnetic disk that can record information of 60 GB or more per 2.5-inch magnetic disk (donut-shaped disk having an outer diameter of 65 mm and an inner diameter of 20 mm). Accordingly, there are required a magnetic disk and a magnetic disk glass substrate that can further improve the information recording density as compared with conventional ones.
Further, with respect to recent hard disk drives, the mobile type has been spreading in place of the conventionally used desktop type. Therefore, there are demand for a magnetic disk and a magnetic disk glass substrate that are excellent in shock resistance as compared with conventional ones.
Moreover, recently, for the purpose of increasing a recording/reproducing region per magnetic disk surface, further reducing the flying height of a magnetic head, and so on, a hard disk drive that performs a start/stop operation by the use of an LUL (Load Unload) system is now spreading in place of a hard disk drive that performs a start/stop operation by the use of a CSS (Contact Start Stop) system.
In the LUL system, while the hard disk drive is stopped, a magnetic head is retreated outside a magnetic disk surface and, at the time of starting, the magnetic head enters over the magnetic disk surface via a ramp and carries out recording/reproduction while flying. When the hard disk drive stops again, the magnetic head is retreated to the outside of the magnetic disk surface via the ramp.
In the LUL system, the magnetic head often interferes with the magnetic disk surface when the hard disk drive starts up. In order to relax such interference, it is preferable that the end portion shape of a magnetic disk glass substrate is controlled to a predetermined shape. By providing the predetermined end portion shape, it is also possible to increase a recording/reproducing region on the magnetic disk surface.
In addition, recently, it is required to provide a hard disk drive at an even reduced price.